The present invention relates to a method and apparatus for storage and/or reproduction of modulated signals, particularly but not exclusively in apparatus for testing radio equipment.
In one conventional method of testing radio equipment, a radio transmitter is connected directly to the equipment to be tested and the radio transmitter generates test signals. Typically, the transmitter is connected to the equipment to be tested via a channel simulator, which simulates effects such as fading, Doppler shift and interference. An example of an RF channel simulator is described in GB-A-2283392. These functions may be built into the transmitter.
This conventional method requires the transmitter and channel simulation equipment to be transported to the test site. Typically, the transmitter and channel simulator are expensive equipment and it is therefore very costly to provide multiple sets of such signal generation equipment if more than one party needs to test their equipment at the same time.
In an alternative conventional method, test signals are broadcast over the air, so that the signal generation equipment does not need to be transported to the receiving party, and multiple receiving parties can test their receiving equipment at the same time. However, this method is not entirely satisfactory, because the signal is degraded between the transmitting and the receiving party in an unpredictable manner, so that the desired characteristics of the test signal may be lost.
Moreover, broadcasting such test signals over the air involves the risk of disclosing confidential information concerning signal characteristics to third parties.
According to one aspect of the present invention, there is provided a method and apparatus for storing radio frequency signals, in which a narrow band of the radio frequency spectrum, having an audio frequency bandwidth, is down converted into the audio frequency range and recorded on an audio-frequency storage medium so that the amplitude, phase and frequency characteristics of the RF spectrum are preserved.
According to another aspect of the present invention, there is provided a method and apparatus for generating radio frequency signals, in which an audio-frequency signal is reproduced from the audio-frequency digital storage medium and is converted into a narrow band of the radio frequency spectrum so as to reproduce the amplitude, phase and frequency characteristics of a radio frequency signal.
In this way, radio frequency signals can be stored and reproduced accurately using inexpensive and commonly available digital audio storage media, such as digital audio tapes, optical or magneto-optical discs or computers having audio processing facilities.
Advantageously, such a method and apparatus can be used to provide test signals for testing the performance of a demodulator and/or a codec. Alternatively, a two-channel digital audio recorder may be used to record RF signals received by a device to be tested on one channel, and the signals generated by the device under test in response to the test signals, on the other channel. Thus, a two-way protocol exchange can be stored conveniently for later analysis.
In another advantageous embodiment, the digital audio recorder may be used to record burst signals for analysis in non-real time. The recording of burst signals may be triggered by the detection of such burst signals.
In another advantageous embodiment, the reproduced test signals are simultaneously supplied to multiple demodulators or signal analysers, for teaching purposes.
In another advantageous embodiment, multiple copies of the recorded signal are distributed to local broadcasting stations for subsequent broadcast. In this way, broadcast programmes can be disseminated in a convenient form, without requiring complex modulating equipment at the broadcast station.